Datasheet

®
22
PCM3501
OUTPUT FILTER CIRCUITS FOR THE DAC
The PCM3501’s DAC uses delta-sigma conversion tech-
niques. It uses oversampling and noise shaping to improve
in-band (f = f
S
/2) signal-to-noise performance at the expense
of increased out-of-band noise. The DAC output must be
low-pass filtered to attenuate the out-of-band noise to a
reasonable level.
The PCM3501 includes a low-pass filter in the on-chip
output amplifier circuit. The frequency response for this
filter is shown in Figure 20. Although this filter helps to
lower the out-of-band noise, it is not adequate for many
applications. This is especially true for applications where
the sampling frequency is below 16kHz, since the out-of-
band noise above f
S
/2 is in the audio spectrum. An external
filter circuit, either passive or active, is required to provide
additional attenuation of the out-of-band noise. The low-
pass filter order will be dependent upon the out-of-band
noise requirements for a particular system. Generally, a 2nd-
order or better low-pass circuit will be required, with the
cut-off frequency set to f
S
/2 or less.
Burr-Brown Application Bulletin AB-034 provides infor-
mation for designing both Multiple Feedback and Sallen-
Key active filter circuits using software available from Burr-
Brown’s web site. Another excellent reference for both
passive and active filter design is the “Electronic Filter
Design Handbook, Third Edition” by Williams and Taylor,
published by McGraw-Hill.
ON-CHIP ANALOG FRONT END FOR THE ADC
The PCM3501 A/D converter includes a fully differential
input delta-sigma modulator. In order to simplify external
circuitry, an analog front end (AFE) circuit has been in-
cluded on the PCM3501 just prior to the modulator. The
AFE circuit is shown in Figure 21.
FIGURE 20. DAC Output Amplifier Filter Response.
FIGURE 21. On-Chip AFE Circuit for the ADC.
OUTPUT FILTER
STOPBAND FREQUENCY RESPONSE
Frequency (Hz)
Amplitude (dB)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
100 1k 10k 100k 1M 10M
OUTPUT FILTER
PASSBAND FREQUENCY RESPONSE
Frequency (Hz)
Amplitude (dB)
0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–0.8
–0.9
–1.0
100101 1k 10k 100k
50kΩ
V
IN+
4
1
2
3
V
COM
V
REF
1
V
REF
2
Delta-Sigma
Modulator
(+)
(–)
Reference
+
+
0.1µF
+
+
V
IN–
+
0.1µF
5
1µF
1µF
50kΩ
1µF